This invention relates to thin films of silicon useful in such applications as diodes, solar cells, image pick-up tubes, electrographic printers for readers and the like.
To suit their intended uses, thin films of silicon have been employed alone or generally in the form of p-i-n or p-n junction devices. Those devices are usually made in a process combining two or three film-forming steps, viz., by first forming a p-type thin film of silicon doped, e.g., with boron (B) as an impurity element, in a plasma atmosphere by glow discharge, and then allowing to grow thereon either an undoped i-type thin film of silicon as an active layer and an overlying phosphorus (P) doped n-type thin film of silicon or the latter directly without the intermediate layer. Alternatively, in a similar but reversed way, a thin film to constitute an n-type layer is formed first and then either i- and p-type layers are deposited thereon or the p-type layer is deposited directly on the n-type layer. A disadvantage common to the devices made by the conventional film-forming techniques is that, in the course of deposition of another layer over a grown layer, the impurity in the underlying (already grown) film tends to be released in the plasma atmosphere and gain entrance into the film being formed thereon. Consequently, as compared with a film deposited on an impurity-free substrate, the film that has grown up on an impurity-containing backing shows decreases in photoelectric and dark electric conductivities. This is particularly true with a p-i-n junction semiconductor device, consisting of a substrate and p-, i-, and n-type layers deposited thereon in order of mention, for fabrication into a solar cell. Ingress of the dopant impurity from the p-type to the i-type film lowers the photoelectric and dark electric conductivities and renders it impossible to provide good junctions. On the other hand, a device formed in the backward order, i.e., of the substrate, n-, i-, and p-type layers, cannot obtain adequate open voltage because the partial transfer of the impurity from the n-type to the i-type film causes a shift of the Fermi level position. Altogether, the aforesaid drawbacks lead to a decline in the photoenergy conversion efficiency, and those junctions in fact affect adversely the performance of the solar cells and other end products incorporating the same.
The present inventors have now found that, if a p- or n-type thin film of silicon doped with an impurity element is placed under a plasma discharge of the gas of at least one element chosen from among fluorine, chlorine, bromine, iodine, and hydrogen, the p- or n-type film will show a decrease in its impurity concentration in the portion from the surface to a depth of 5000 .ANG., and the dangling bonds that have resulted from the removal of the impurity from the p- or n-type silicon film are replaced by the plasma discharge gas, thus forming a barrier layer to avoid further release of the impurity from the silicon film in the plasma atmosphere.
It has also been found that the degree of impurity decrease in the thin film of silicon and the depth of film portion in which the impurity decrease takes place can be varied over broad ranges by adjusting the pressure during the plasma discharge inside the vacuum vessel, discharge time, and power density for discharge.
The pressure during the discharge, one of the essential parameters for practicing the method of the invention, is controlled desirably within a range from 1.5.times.10.sup.-2 to 3 torrs. If the discharge pressure is less than 1.5.times.10.sup.-2 torr, the flow inside the vacuum vessel will become a diffusion or nonlaminar one with an increasing possibility of the once released impurity finding entrance back into the silicon film substrate. Thus, in order to keep the flow inside the vessel viscous laminar, the pressure should be not less than 1.5.times.10.sup.-2 torr. The upper limit of 3 torrs is determined primarily by reason of apparatus factors, so as to prevent any discharge between the electrodes and the earth shields. The discharge power density varies depending on the properties of the plasma gas to be employed but a value between 0.5 and 50 W/cm.sup.2 is suitable. Under these conditions the discharge time is variable as desired within a range from one second to five hours. With regard to the relation between the discharge power density and the discharge time, it may generally be said that the discharge power density influences the depth of the film portion in which the impurity element originally added as a dopant decreases, while the discharge time influences the decrease in the originally added impurity concentration.
The flow rate of the plasma element gas into the vacuum vessel to bring a plasma state must be so fixed as to stably maintain the plasma state. Experiments have indicated that a flow rate ranging from 0.5 to 100 SCCM gives a good result.
The present invention is predicated upon the aforedescribed novel findings. Briefly, the method of producing thin films according to this invention is distinguishedly characterized in that a p-type or n-type thin film of silicon is placed in a state of plasma discharge of the gas of at least one element chosen from among fluorine, chlorine, bromine, iodine, and hydrogen, whereby the impurity concentration in the portion of the p- or n-type silicon film to a desired depth down to 5000 .ANG. from the surface is decreased and the dangling bonds that have resulted from the impurity removal are replaced by the plasma discharge gas.
Thus, the p-type or n-type thin film of silicon produced in accordance with the invention, when subjected to a low-power plasma discharge in the next process stage for deposition thereon, e.g., of an i-type film layer, will not release its impurity into the latter layer.
When a film is caused to grow in a plasma atmosphere on an n-type or p-type thin film substrate formed in a conventional way, the additional film will contain more than 10.sup.16 atoms/cm.sup.3 of the impurity from the substrate film. In contrast with that, when a film is grown likewise on a similar substrate formed by the method of the invention, the release of the impurity from the substrate film to the overlying film can be controlled to be a maximum of 10.sup.16 atoms/cm.sup.3.
Therefore, it is a primary object of this invention to provide a p-type or n-type thin film of silicon which, upon exposure to a given plasma atmosphere, will release no (or little, if any) impurity with which it is doped to the exterior.
Another object of the invention is to provide thin films of silicon useful in fabricating solar cells, image pick up tubes, electrographic printers for readers, diodes, etc., with good photoelectric and dark electric conductivities.
Other objects and advantages of the invention will become more apparent when reference is made to the following description and accompanying drawings.